A suicide bomber killed at least 14 and wounded eight in an attack on a minibus carrying Nepalese security contractors in the Afghan capital, Kabul, early on Monday, officials said.

A Reuters witness saw several apparently dead victims and at least two wounded being carried out of the yellow bus, while police and emergency service vehicles surrounded the scene in the Banae district of the city.

The attack was the latest in a recent surge of violence that highlights the challenges faced by the Afghan government in Kabul and its Western backers as Washington slowly draws down its remaining troops despite a persistent insurgency.

Interior ministry spokesman Sediq Sediqqi said on Twitter that 14 people had been killed and eight wounded. Police were working to identify the victims, he said.

The casualties appeared to include both Afghans and Nepalese security contractors, Kabul police chief Abdul Rahman Rahimi said.

He said the suicide bomber had waited near the compound housing the security contractors and struck as the vehicle moved through early morning traffic. As well as the bus passengers, several people in an adjacent market were also wounded.

Indian Prime Minister Narendra Modi condemned the attack on the minibus that was carrying Nepalese security guards in Kabul. He said India will provide all assistance to Nepal government in this hour of tragedy.

"We strongly condemn the horrible tragedy in Kabul. Our deep condolences to the people and governments of Afghanistan and Nepal on the loss of innocent lives," he said in a tweet.

"We are taking steps to provide all relevant assistance to the Nepal government in this hour of tragedy," PM Modi said.

Fourteen Nepalese security guards were killed when a Taliban suicide bomber targetted their minibus in the Afghan capital.

There was no immediate claim of responsibility for the blast, which occurred during Ramadan, Islam's holy month.

It underlined how serious the security threat facing Afghanistan remains since the death of former Taliban leader Mullah Akhtar Mansour in a US drone strike last month.

The blast follows a deadly suicide attack on a bus carrying justice ministry staff near Kabul last month and a separate attack on a court in the central city of Ghazni on June 1.

The Taliban claimed both those attacks in revenge for the execution of six Taliban prisoners.